CUPERTINO -- Apple said Tuesday that its cloud-based system for storing users' content was not at fault for a widely publicized theft of celebrities' personal photos, based on a preliminary investigation.

The world's most valuable company said in a media advisory Tuesday morning that it had been investigating the theft of nude photos of actress Jennifer Lawrence and other celebrities for 40 hours and found that the users' individual accounts had been targeted and hacked.

"None of the cases we have investigated has resulted from any breach in any of Apple's systems including iCloud or Find my iPhone," the company's statement read. "We are continuing to work with law enforcement to help identify the criminals involved."

The leak of nude photos of several celebrities caused an initial uproar after the images -- some of which had reportedly been deleted before being found on iCloud and retrieved -- began showing up on the Internet on Sunday. Lawrence, whose publicist called posting the pictures "a flagrant violation of privacy," and other actresses lashed out Monday and the FBI began investigating the incident.

Actress Kirsten Dunst, another victim, tweeted on Monday afternoon, "Thank you iCloud," followed by emoji meant to spell out a denigrating phrase.

Actress Mary Elizabeth Winstead also took to Twitter, confirming that nude pictures of here were taken long ago and deleted.

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"To those of you looking at photos I took with my husband years ago in the privacy of our home, hope you feel great about yourselves," she tweeted.

Apple's statement refers to the hack as "very targeted attack on user names, passwords and security questions, a practice that has become all too common on the Internet."

That language infers that the hackers used a tactic commonly referred to as social engineering to obtain the photos, which involves using some information known about a subject -- such as answers to possible security questions -- to obtain more knowledge, such as passwords. A similar tactic was used by hackers to obtain control of Wired journalist Mat Honan's Apple account in a previous widely publicized attack in 2012.

Apple suggested in its note that users turn on its two-factor authentication to deter such attacks in the future. That process requires a user attempting to enter an account without a password have access to another device affiliated with an account, commonly a smartphone.

Despite negative publicity surrounding the hack, Apple again hit record highs on Wall Street, the seventh trading session in the past nine in which the Cupertino company hit an all-time intraday high. Shares moved as high as $103.74 by noon Pacific time, when the stock was trading at $103.20, a 0.7 percent daily increase.